如何使用Angularfire动态访问firebase数组的子记录

I am trying to implement a accordion using angularfire . I am able to retrieve the top level list ("A1","D1","R1")for display but I am unable to figure out how to retrieve the child for each accordion tab that is selected. For Eg if I select "D1", it should open up and display "C1", "H1". I am trying to implement a accordion using angu